1. Field of the Invention
This invention relates to a method of providing heat to the coking zone of a fluid coking process.
2. Description of the Prior Art
Fluid coking is a well known process which may be carried out with or without recycle of the heavier portion of the fluid coking zone effluent. See, for example, U.S. Pat. No. 2,881,130, the teachings of which are hereby incorporated by reference.
Integrated fluid coking and gasification processes are also known. See, for example, U.S. Pat. Nos. 3,661,543; 3,702,516; 3,759,676, the teachings of which are hereby incorporated by reference.
It is also known to utilize light hydrocarbons to supplement or be used as fluidizing gas in the coking zone and thereby reduce the steam requirements of the process, see, for example, U.S. Pat. Nos. 2,636,844; 2,734,852; 2,863,823 and 3,671,424.
U.S. Pat. No. 2,661,324 discloses a coking process in which heated fine coke particles supply the major portion of the heat requirements of the coking zone. In the embodiment shown in FIG. 1, additionally, a high temperature fluid medium is introduced into the lower portion of the coking chamber for stripping the coke.
In the conventional fluid coking process and in the integrated fluid coking and gasification processes, the major portion of the heat requirements of the coking zone are provided by a recycled stream of hot solids, generally coke particles which are heated by the exothermic reaction of oxygen with coke in the burner of a conventional fluid coker or in the gasifier of the integrated coking and gasification process. It has now been found advantageous to utilize a hot vaporous hydrocarbon stream to provide the heat requirements of the coking zone, instead of utilizing recycled solids. The benefits are derived from minimizing or eliminating certain requirements which are associated with the recycled solids stream, and from other effects which are associated with the recycled hydrocarbon stream. The main advantages are summarized below and will be apparent in the description which follows.
Some lighter-than-coke hydrocarbon material is transported with the recycled coke stream from the coking zone into a conventional coke burner or into the heater of an integrated coking and gasification unit. This constitutes a loss of potential overhead product from the coking zone, which will be recovered when the light hydrocarbon recycle heating method of the present invention is used. Similarly, the recycled coke transports some gases containing nitrogen and carbon oxides into the coking reactor from which they pass overhead and contaminate the coker products. This product contamination is avoided with the light hydrocarbon recycle heating system of the present invention.
Recycling the solids stream requires that a substantial quantity of steam, or other lift gas, be used in the upflowing portion of each solids circuit. There are energy debits associated with the supply of these lift gas streams, and with the heating and subsequent cooling of the lift gas within the process system. These debits are eliminated with the recycle hydrocarbon heating system of the present invention.
The recycled hydrocarbon moles will be sufficient to maintain good fluidization throughout the entire coking zone and will eliminate the need for the reactor stripping and fluidization steam employed in conventional commercial units heated by recycled solids.